A cold high pressure air mass will be reinforced with a second push of very cold air Thursday with gusty west to northwest winds expected into the weekend. However a pattern change early next week may lead to more "normal" late fall conditions, which often sees the establishment of slow-moving high pressure regimes centered over New England. These highs give a cool shallow east to southeasterly flow over northern Illinois and the Upper Midwest at ground level, while southwesterly flow from developing low pressure in the central and northern Plains flows into Illinois. This type of pattern lead to the leaf-burning bans in effect over many metropolitan areas.
Warmth aloft can trap rising pollutants
Warm air pushed aloft over cooler air creates a broad scale stable inversion (temperature increase with height) that allows pollutants such as smoke, industrial and car exhausts to collect in the air over the city and suburbs. As long as the temperature of the pollutant is warmer than it's surroundings, it will rise; the rising contaminated air cools at a rate of 5.5(degrees) per 1,000 feet and soon reaches a point aloft with the air above is warmer, forcing the pollutant to stratify and slowly collect under the warm "lid" above. Moist flow off Lake Michigan enhances formation of lower stratus clouds at the same time.
Tom Skilling is chief meteorologist at WGN-TV. His forecasts can be seen Monday through Friday on WGN-TV News at noon and 9 p.m.
WGN-TV meteorologists Steve Kahn, Richard Koeneman and Paul Dailey plus weather producer Bill Snyder contribute to this page.